Christopher is 4 times as old as Jessica and is also 15 years older than Jessica. How old is Christopher?
Answer: We can use the given information to write down two equations that describe the ages of Christopher and Jessica. Let Christopher's current age be $c$ and Jessica's current age be $j$ $c = 4j$ $c = j + 15$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $c$ is to solve the second equation for $j$ and substitute that value into the first equation. Solving our second equation for $j$ , we get: $j = c - 15$ . Substituting this into our first equation, we get the equation: $c = 4$ $(c - 15)$ which combines the information about $c$ from both of our original equations. Simplifying the right side of this equation, we get: $c = 4c - 60$ Solving for $c$ , we get: $3 c = 60$ $c = 20$.